06/2003 - EtherCAT

ethercat.org

06/2003 - EtherCAT

6 title

Beckhoff EtherCAT – Ethernet for Control Automation Technology

Real-time Ethernet:

Ultra high speed right up to the I/O

It was presented for the first time at the Hanover exhibition in Germany, and provided a talking point amongst automation

engineers: EtherCAT – the new, real-time Ethernet network from Beckhoff. EtherCAT is remarkable for its exceptional performance, extremely

easy wiring, and its openness to other protocols. Where conventional fieldbus systems come up against their limits, the real-time Ethernet

system is setting new standards.

The properties speak for themselves: 1000 I/O in 30 µs, optionally twisted pair cable

or optical fiber and, thanks to Ethernet and Internet technologies, optimum

vertical integration. EtherCAT gives you the option of using the classic more expensive

star topology or a simple low cost line structure - no expensive infrastructure

components are required. EtherCAT uses very cost-effective standard

Ethernet cards (NIC) while other real-time-Ethernet approaches require special

and expensive cards in the controller.

The EtherCAT operating principle

With EtherCAT technology, Beckhoff overcomes the system limitations of other

Ethernet solutions:The Ethernet packet is no longer received, then interpreted and

copied as process data at every connection. The newly developed FMMU (fieldbus

memory management unit) in each I/O terminal reads the data addressed to

it, while the telegram continues through the device. Similarly, input data are inserted

while the telegram passes through. The telegrams are only delayed by a

few nanoseconds.

Other Ethernet approaches cannot match the EtherCAT real-time capability. One

approach includes disabling the CSMA/CD access procedure via higher level protocol

layers and replacing it with a time slice procedure or a polling procedure.

Another approach uses special switches that distribute Ethernet packets in a precisely

controlled timely manner. These other approaches are all capable, to a certain

degree, of quickly and accurately transferring data from the controller to the

Ethernet node. However, these other approaches are limited because of delays

from the Ethernet node to the actual I/O or drive controllers.The other approaches

require a sub bus especially when using modular I/O systems. The other Ethernet

approaches are made faster through synchronization of the sub bus system much

like Beckhoff has done in the past with other existing fieldbus networks. However,

the synchronization creates small delays to the communication bus that cannot

be avoided. Beckhoff takes the next step in technology using the FMMU technology

in EtherCAT.


Full duplex Ethernet in the ring, one telegram for many devices:

The EtherCAT system architecture increases the “communication efficiency”.

Maximum flexibility for wiring: with or without switch, line or tree topologies can

be freely selected and combined. Cost-effective twisted pair cable, selection of the

transfer physics depending on requirements. Address assignment is automatic;

no IP address setting is required.

Ethernet up to the terminal – complete continuity

The Ethernet backplane for the I/O modules is called E-bus. The E-bus transfers

the data from one I/O point to another using a different electrical signal but not

changing the Ethernet data. The first Ethernet node, called the Bus Coupler, converts

the electrical signal from standard twisted pair or fiber optics to E-bus. The

signal is converted to E-bus to meet electronic terminal block electrical signal re-

7 title

quirements. The signal type within the terminal block (E-bus) is also suitable for

transfer via a twisted pair line over short distances (up to 10 m). The terminal

block can thus be extended very cost-efficiently. Subsequent conversion to Ethernet

is possible at any time since the Ethernet data is never changed.

On the control side, very inexpensive, commercially available standard network

interface cards (NIC) are used as hardware in the controller. The cards offered by

Beckhoff bundle up to 4 Ethernet channels on one PCI slot and are based on the

same architecture. The common feature of these interface cards is data transfer

to the PC via DMA (direct memory access), i.e. no CPU capacity is taken up for

the network access.

The NIC cards use the TwinCAT Y driver which operates seamlessly with the software

operating system and the real-time system. This means the TwinCAT Y driver

functions as a compatible network driver, and additionally as a TwinCAT Ethernet

fieldbus card. The real-time system Ethernet frames have priority over the

general operating system frames using an internal prioritization system. The general

operating system’s Ethernet frames, such as print spooling, Internet, and

mail, are transmitted in the “gaps” if sufficient time is available.

At the receiving end, all the Ethernet frames received are examined by the Twin-

CAT I/O system, and those with real-time relevance are filtered out. All other

frames are passed on to the operating system after examination, outside the context

of the real-time system.

Since the Ethernet functionality of the operating system is fully maintained, all

operating system-compatible protocols can be operated in parallel on the same

physical network. This not only includes standard IT protocols such as TCP/IP,

HTTP, FTP or SOAP, but also practically all Industrial Ethernet protocols such as

Modbus TCP, ProfiNet or EthernetIP.

Optimized protocol directly within the Ethernet frame

The EtherCAT protocol uses a special Ether-type inside the Ethernet Frame. The

Ether type allows transport of control data directly within the Ethernet frame

without redefining the standard Ethernet frame. The frame may consist of several

sub-telegrams, each serving a particular memory area of the logical process images

that can be up to 4 gigabytes in size. Addressing of the Ethernet terminals

can be in any order because the data sequence is independent of the physical order.

Broadcast, Multicast and communication between slaves are possible. Transfer

directly in the Ethernet frame is used in cases where EtherCAT components

are operated with TwinCAT and in the same subnet as the control computer.

However, EtherCAT applications are not limited to TwinCAT as the control system:

EtherCAT UDP packs the EtherCAT protocol into UDP/IP datagrams. This enables

any control with Ethernet protocol stack to address EtherCAT systems. Even communication

across routers into other subnets is possible. In this variant, system

performance obviously depends on the real-time characteristics of the control and

its Ethernet protocol implementation. The response times of the EtherCAT network

itself are hardly restricted at all: The UDP datagram only has to be unpacked

in the first station.


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Protocol processing completely in hardware: The protocol ASICs are flexibly

configurable. Process interface from 2 bit to 64 kB.

Ethernet

Ethernet

Branch

Coupler Terminal I/O E-Bus Terminal Intelligent

E-Bus Terminal

converts transmission

physics from Ethernet

to E-Bus

Power Supply system:

24 V DC

2 … 4 bit wide I/O data

interface to E-Bus

up to 4 kByte wide

I/O-data and parameter

interface to E-Bus

Power Supply I/O:

5 ... 230 V DC

Power

Supply

E-Bus

1000 I/Os in 30 µs | 200 analog I/Os in 50 µs | 100 axes in 100 µs

EtherCAT reaches new dimensions in network performance. Thanks to FMMU in

the terminal and DMA access to the network card in the master, the complete

protocol processing takes place within hardware and is thus independent of the

run-time of protocol stacks, CPU performance or software implementation. The

update time for 1000 I/Os is only 30 µs - including terminal cycle time. Up to 1486

bytes of process data can be exchanged with a single Ethernet frame - this is

equivalent to almost 12000 digital inputs and outputs. The transfer of this data

quantity only takes 300 µs.

The communication with 100 servo axes only takes 100 µs. During this time, all

axes are provided with set values and control data and report their actual position

and status. The distributed clock technique enables the axes to be synchronized

with a deviation of significantly less than 1 microsecond.

The extremely high performance of the EtherCAT technology enables control concepts

that could not be realized with classic fieldbus systems. For example, the

Ethernet system can now not only deal with velocity control, but also with the

current (torque) control of distributed drives. The tremendous bandwidth enables

status information to be transferred with each data item. With EtherCAT, a communication

technology is available that matches the superior computing capacity

of modern Industrial PCs. The bus system is no longer the “bottleneck” of the

control concept. Distributed I/Os are recorded faster than is possible with most

local I/O interfaces. The EtherCAT technology principle is scalable and not bound

to the baud rate of 100 MBaud – extension to GB Ethernet is possible.

Topology – maximum flexibility

Line, tree or star: EtherCAT supports almost any topology. The bus or line structure

known from the fieldbusses thus also becomes available for Ethernet. Particularly

useful for system wiring is the combination of line and branches or stubs:

+

-

PE

Power Supply Power Supply

E-Bus

BE1000 ASIC BE1000 ASIC

Opto Isolation

Digital I/O

+

-

PE

DC/DC

Conv.

SSI

µC

SSI

Analog I/O

EPROM

Opto

Isolation

Power Supply

E-Bus

BE1100 ASIC

DPRAM

E-Bus

+

-

PE

Fieldbus Master

at the E-Bus

4 … 64 kByte wide

process data and

parameter interface

16 bit

µC

EPROM RAM

Opto Isolation

Fieldbus Transceiver

E-Bus

End Terminal

with E-Bus

extension

interface

E-Bus

The required interfaces exist on the couplers; no additional switches are required.

Naturally, the classic switch-based Ethernet star topology can also be used.

Wiring flexibility is further maximized through the choice of different cables. Flexible

and inexpensive standard Ethernet patch cables transfer the signals optionally

in Ethernet mode (100Base-TX) or in E-bus signal representation. Plastic fiber

optics (PFO) can be used in special applications. The complete bandwidth of the

Ethernet network – such as different fiber optic and copper cables – can be used

in combination with switches or media converters.

Fast Ethernet or E-bus can be selected based on distance requirements. The Fast

Ethernet physics enables a cable length of 100 m between devices while the Ebus

line is intended for distances of up to 10 m. The size of the network is almost

unlimited since up to 65535 devices can be connected.

TwinCAT Y driver:

operating system-compatible


EtherCAT instead of PCI

The central PC becomes smaller and more cost-effective because additional

slots are not needed for interface cards since the onboard Ethernet port can be

used. With increasing miniaturization of the PC components, the physical size of

Industrial PCs is increasingly determined by the number of required slots. The

bandwidth of Fast Ethernet, together with the data width of the EtherCAT communication

hardware (FMMU chip) enables new directions: Interfaces that are

conventionally located in the IPC are transferred to intelligent interface terminals

at the EtherCAT. Apart from the decentralized I/Os, axes and control units,

complex systems such as fieldbus masters, fast serial interfaces, gateways and

other communication interfaces can be addressed. Even further Ethernet devices

without restriction on protocol variants can be connected via decentralized

“hub terminals”. The central IPC becomes smaller and therefore more costeffective,

an Ethernet interface is sufficient for the complete communication

with the periphery.

From Master

To Master

FMMU (fieldbus memory management unit):

Telegram processing completely in hardware

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Precise synchronization through distributed clock

Accurate synchronization is particularly important in cases where widely distributed

processes require simultaneous actions. This may be the case, for example,

in applications where several servo axes carry out coordinated movements simultaneously.

The most powerful approach for synchronization is the accurate alignment of distributed

clocks, as described in the new IEEE 1588 standard. In contrast to fully

synchronous communication, where synchronization quality suffers immediately

in the event of a communication fault, distributed aligned clocks have a high degree

of tolerance from possible fault-related delays within the communication

system.

With EtherCAT, the data exchange is completely hardware based on “mother”

and “daughter” clocks. Each clock can simply and accurately determine the

other clocks’ run-time offset because the communication utilizes a logical and

full-duplex Fast Ethernet physical ring structure. The distributed clocks are adjusted

based on this value, which means that a very precise network-wide timebase

with a jitter of significantly less then 1 microsecond is available.

However, high-resolution distributed clocks are not only used for synchronization,

but can also provide accurate information about the local timing of the data acquisition.

For example, controls frequently calculate velocities from sequentially

measured positions. Particularly with very short sampling times, even a small

temporal jitter in the displacement measurement leads to large step changes in

velocity. With EtherCAT, Beckhoff introduces new, expanded data types (timestamp

data type, oversampling data type). The local time is linked to the measured

value with a resolution of up to 10 ns, which is made possible by the large

bandwidth offered by Ethernet. The accuracy of a velocity calculation then no

longer depends on the jitter of the communication system. It is orders of magnitude

better than that of measuring techniques based on jitter-free communication.

Hot connect and diagnosis

The Hot Connect function enables parts of the network to be linked and decoupled

or reconfigured “on the fly”; offering flexible responses to changing configurations.

Many applications require a change in I/O configuration during operation.

Examples are processing centers with changing, sensor-equipped tool systems

or transfer devices with intelligent, flexible workpiece carriers. The protocol

structure of the EtherCAT system takes account of these requirements.

Special attention was paid to exemplary diagnostic features during the development

of EtherCAT. The Beckhoff comprehensive experience with fieldbus systems

shows that availability and commissioning times crucially depend on the diagnostic

capability. Only faults that are detected quickly and accurately and located

unambiguously can be rectified quickly.

During commissioning, the actual configuration of the I/O terminals is checked for

consistency with the specified configuration. The topology should also match the

configuration. I/O verification is possible during start-up and also via automatic


The process image allocation is freely configurable. Data are copied directly in the I/O

terminal to the desired location within the process image: no additional mapping is required.

Very large address space of 4 GB.

configuration upload because of the built-in topology recognition.

Bit faults during the data transfer are reliably detected through evaluation of the

32 bit CRC checksum, which has a minimum hamming distance of 4. The Ether-

CAT protocol, transfer physics and topology enables quality monitoring of each

individual transmission segment. The automatic evaluation of the associated error

counters enables precise localization of critical network sections. Gradual or

changing sources of error such as EMC influences, defective push-in connectors

or cable damage are detected and located, even if they do not yet overstrain the

self-healing capacity of the network.

EtherCAT Highlights

Performance

| 256 digital I/Os in 12 µs

| 1000 digital I/Os in 30 µs

| 200 analog I/Os (16 bit)

in 50 µs, corresponding to

20 kHz sampling rate

| 100 servo axes in 100 µs

| 12000 digital I/Os in 350 µs

| Throughput: 10 kB/ms, distributed

to 1,500 devices

Topology

| Line, tree or star topology

| Up to 65,535 devices

| Network size:

almost unlimited (> 500 km)

| Operation with or without

switches

| Cost-effective cabling: standard

Ethernet patch cable (CAT5)

| Twisted pair physical layer:

| Ethernet 100BASE-TX, up to

100 m between 2 devices

| E-bus, industrial grade Ethernet,

up to 10 m between 2 devices

| Optional fiber optic cable from

50 to 2000 m

| Hot connect/disconnect of

bus segments

Address space

| Network-wide process image:

4 GB

| Device process image:

2 bit to 64 kB

| Address allocation:

freely configurable

| Device address selection:

automatically via software

Ethernet Terminals

The existing wide range of K-bus I/O terminals from the proven Beckhoff Bus Terminal

line can be networked with EtherCAT. The range includes appropriate bus

couplers which are the network interface for the modular I/O terminals. This ensures

compatibility and continuity with the existing system. Existing and future

investments are protected.

Protocol

| Optimized protocol directly within

the Ethernet frame

| Fully hardware-implemented

| For routing and socket interface:

UDP datagram

| Processing while passing

| Distributed clock for accurate

synchronization

| Time stamp data types for resolution

in the nanosecond range

| Oversampling data types for

high-resolution measurements

Diagnostic

| Breaking point detection

| Continuous “quality of line”

measurement enables accurate

localization of transmission faults

10 title

Openness

Beckhoff has taken every effort to

ensure EtherCAT technology is fully

Ethernet-compatible and truly open.

The protocol tolerates other Ethernet-based

services and protocols on

the same physical network – usually

even with minimum loss of performance.

There is no restriction on

the type of Ethernet device that can

be connected within the EtherCAT

strand via a hub terminal. Devices

with fieldbus interface are integrated

via EtherCAT fieldbus master

terminals. The UDP protocol variant

can be implemented on each socket

interface. Finally, the intention is to

disclose the technology once the

development work is completed.

Interfaces

| Hub terminal for standard

Ethernet devices

| Fieldbus terminals for fieldbus

devices

| Decentralized serial interfaces

| Communication gateways

Openness

| Fully Ethernet-compatible

| Operation with switches and

routers possible

| Mixed operation with other

protocols also possible

| Internet technologies

(web server, FTP, etc.)

| Compatible with the

existing Bus Terminal range

| Disclosure in preparation

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